This invention claims the priority of Japanese Patent Application No. 171873/1994 filed Jun. 29, 1994, now abandonment which is incorporated herein by reference.
Instead of utilizing electric cables, optoelectronic achieves transmission through optical fibers. Optoelectronic transmission is promising for increasing the number of channels and receiving areas. The optoelectronic communication system adopts a laser as a light source because of the high stability of the light wavelength. The laser converts electric signals into light signals in the system. The optoelectronic system employs amplitude-modulation (AM) for transmitting information riding thereon instead of frequency modulation (FM) or phase-modulation (PM). Namely, the power of the light beams emitted from the laser is changed in an analog manner in accordance with the signals. There are two ways for modulating the light power. An inner modulation varies the injection current of the laser itself. An outer modulation utilizes a modulator for varying the power of the light beams. The light beams emitted from the laser are introduced into an optical fiber. A transmitting station gathers a plurality of signals from different channels having different carrier frequencies in the transmitting station. The sum of the signals is transmitted through an optical fiber from the transmitting station to receivers.
The single optical fiber can transmit a plurality of signals of different frequencies which are individually modulated in an analog manner. Every channel is allocated to one of the transmitting frequencies. Thus the number of the frequencies is equal to the number of the channels. The difference of the frequency prevents the signals of different channels from mingling with each other. A receiver obtains the signal of the desired channel by receiving the optical signal from the fiber, converting the optical signals to electrical signals by a photodiode, and selecting (by a tuner) one channel which the listener wishes by the difference of the frequencies.
One optical fiber is sometimes assigned to several tens to several hundreds of channels. For transmitting many channels, the effect of non-linearlity should be reduced in the transmitting device and the receiving devices. The non-linearlity generates harmonics. The harmonics cause a cross-talk between one channel A and another channel B generating harmonic whose frequency is similar to the frequency of the channel A. Furthermore, beats having a frequency equal to the difference of two frequencies are generated by various harmonics. The beat sometimes incurs unpleasant noises in receivers. Therefore, the non-linearlity causes the noise or the cross-talk between two different channels. What demands the linearlity is all the components of the broadcasting station and the receivers, that is, the laser diode module (LD) of the transmitting station, the photodiodes (PD) of the receiving sets, optical fibers and so on. The present invention addresses the problem of the non-linearlity of the photodiode modules.
The problem of distortion is explained now. "S" denotes an input signal. "Q" designates an output converted by the photodiode from the input signal S. The output Q can be expanded to Q=aS+bS.sup.2 . . . . In this expansion, the second term b induces the second order distortion of signal. The square term S.sup.2 generates the second harmonics. When two signals are introduced into the photodiode, a sum frequency and a difference frequency are yielded by the function of the square term S.sup.2. The second order distortion is estimated by the quantity obtained by dividing the second harmonic by the fundamental (first order) signal, taking logarithms of the quotient and multiplying it by 20. Thus the second order distortion is denoted by 20 log (bS/a). The quantity is obtained by inputting two signals S and T with different frequencies, and measuring the power of the sum-frequency or the difference-frequency originated from the input signals. The power P of the sum-frequency or the difference-frequency is designated by dBc (=20 logP). The power W of the original signal S or T is also measured and expressed by dBc(=20 logW). The second distortion is given by subtracting the latter 20 logW from the former 20 logP. The second order distortion 20 log(P/W) is designated by IMD.sub.2. The third order term S.sup.3 bears the third order distortion IMD.sub.3. However, higher order distortions than the second are, in general, too weak to cause the problems. Accordingly, the present object is the second order distortion.
For example, InGaAs pin photodiodes are used as an analog photodiode (PD) for receiving the signals of the present optoelectronic communication which adopts the light of a wavelength between 1.0 .mu.m and 1.3 .mu.m. The InGaAs designates the component of the active layer which is deposited via some other layers on an InP substrate. Fujitsu FID13SK81R-AL, an example of the InGaAs photodiodes, ensures the sensitivity higher than 0.8 A/W, the second order distortion IMD.sub.2 less than -75 dBc and the response higher than 1.5 GHz at a 1.3 .mu.m wavelength. The present optoelectronic communication requires the second order distortion less than -75 dBc for analog photodetectors. An example of a conventional photodiode is explained by referring FIG. 7. A package (1) has pins (2), a photodiode chip (3) and a submount (4). The photodiode chip (3) is fitted on the submount (4) which is an insulator plate. A cap (6) with a ball lens (5) covers the package (1). Dry nitrogen gas supplied to the inner space of the cap is sealed. A ferrule (9) holds an end of a single-mode fiber (8). The end of the ferrule (9) together with the fiber (8) are slantingly cut preferably at 8 degrees for preventing the reflected light from returning to the fiber. The ferrule (9) is inserted into a hole of a ferrule holder (10). The holder (10) is positioned and welded to the upper surface of the package (1). The single-mode fiber (8) is drawn out of the ferrule (9). The other end of the fiber (8) communicates with a connector (not shown in the figures). This photodiode module is called a pig-tail type, because a definite length of a fiber projects from the top like a tail of a pig.
For example, a pig-tail type photodetector module of a second order distortion between -80 dBc and -76 dBc was proposed by; Takada, Sato, Yuki, Isaka, Hanamitsu, "Photodetector Module for Analog CATV", Proceedings of the 1990 IEICE Fall Conference V, B-734, p4-69. (IEICE: The Institute of Electronics Information and Communication Engineers)